1. Field of the Invention
This invention relates generally to the use of visible reference lines during sporting or entertainment events and, more particularly, to systems employing at least one laser light beam source to generate such visible reference lines.
2. Discussion of the Background Art
To accommodate a sporting event, a series of reference and/or boundary lines may be defined upon a grass-covered surface using, for example, paint, powders, dyes and the like. Such methods of marking are entirely satisfactory so long as the reference lines themselves are static during the entire event. Where the position of a boundary or other line of demarcation changes dynamically during the event, however, markings of the permanent type cannot be used.
In the game of football, for example, a key objective of the team in possession of the ball (i.e., the xe2x80x9coffensexe2x80x9d) is to retain possession of that ball by moving it far enough down the field. Specifically, the offense is given a set of four plays or xe2x80x9cdownsxe2x80x9d to advance the ball by at least ten yards. Each time that distance is reached or exceeded, the offense is said to have crossed a xe2x80x9cfirst downxe2x80x9d line, a new set of downs is earned, and the offense is allowed to continue its advance toward the goal line of the opposing team (i.e., the xe2x80x9cdefensexe2x80x9d). If the offense falls short, however, possession is lost and the two teams reverse their roles. A regulation football field has a length of 100 yards and 53 yards. Thus, by way of example, a team gaining possession of the ball at its own 20 yard line must move the ball a total of eighty yards in order to reach the end zone of the opposing team.
In numerous occasions throughout an average football game, the officials of the game must resort to sideline markers to establish whether the offense has advanced the ball by the required distance. The standard alignment system that is utilized is generally a pair of poles connected by a 30-foot long chain. The relative position of the football is measured by locating a first of these poles at the approximate location of the initial line of scrimmage and moving the second as far forward as possible. Each time this measurement is made, the game must be delayed and the yard markers must be carried from the sidelines to the place on the field where the official has xe2x80x9cspottedxe2x80x9d the ball. Although the game of football has become a relatively complex sport, involving literally hundreds of millions of invested dollars, this time consuming system has remained relatively the same since the conception of the sport.
A number of approaches intended to ameliorate the aforementioned deficiencies have been proposed over the years, but none of them has met with any degree of commercial success. U.S. Pat. No. 3,741,662, entitled xe2x80x9cVISIBLE LINE MARKERxe2x80x9d and issued to Pioch on Jun. 26, 1973, U.S. Pat. No. 3,752,588, entitled xe2x80x9cLASER FOOTBALL FIRST DOWN MEASURING DEVICExe2x80x9d and issued to Chapman on Aug. 14, 1973, and U.S. Pat. No. 4,090,708 entitled xe2x80x9cAPPARATUS FOR MARKING FOOTBALL FIELDSxe2x80x9d and issued to McPeak on May 23, 1978. Each of the aforementioned patents involves the use of lasers for the purpose of marking visible lines of demarcation on an athletic field. One of the principal drawbacks of these systems is the time-consuming and tedious method of operation.
Both Chapman and Pioch involve the use of track mounted, sliding projectors that are located at the sidelines and just a few feet above the field level. The lasers are mounted for oscillation in a vertical plane and the projected low intensity beam developed by each must strike the field at points of reference lying on an imaginary line of demarcation defined by the intersection of the vertical plane with the field surface. Accordingly, it is necessary for the operator to manually position the projector for each reference point established. Like Pioch and Chapman, McPeak discloses the use of a laser assemblies adapted to accommodate sliding movement along the sidelines of a football field. McPeak, however, teaches that two oppositely directed beams should be aimed at a level above (i.e., xe2x80x9cadjacent and parallel toxe2x80x9d) the field surface.
Another drawback associated with the aforementioned systems is that the low-intensity output of these lasers is far too low to be visible by the players, let alone by an audience in, for example, a stadium setting. Indeed, the aforementioned systems are intended for use only in making a first down measurement determination after each close play. As it turns out, players intent on getting the ball past the first yard linexe2x80x94and focused on the sideline markers long enough to be xe2x80x9cblindsidedxe2x80x9d by the defensexe2x80x94have either fumbled the ball or suffered very serious neck and back injuries.
Television networks have recently implemented an image pre-processing system which allows viewers of televised football games to see a so-called xe2x80x9cvirtualxe2x80x9d first down line that digitally projects, in real time, a visible line onto video frames recorded by the television camera, the line being displayed on a viewer""s television set so that it appears to extend between the first down sideline markers. Unfortunately, neither the players, game officials, nor the fans attending such games can actually see this virtual line. It is thus reasonable to conclude that given the rapid and widespread adoption of a virtual visible line marking systemxe2x80x94whose enjoyment is strictly limited to television viewers, it has heretofore been assumed that it would be impossible or impracticable to project a real, visible line onto surfaces like those of athletic fields. Although there are many possible explanations for this conclusion, it is believed by the inventors herein that the poor light scattering properties of grassy surfaces is at least partially to blame. Blades of grass are randomly oriented and tend to scatter incident light in several directions. The inventors herein have discovered that from distances in excess of one hundred feet or so, a single beam of even relatively high intensity (e.g., 40 joules/second) will be reflected in such a way that it cannot be seen from most camera or fan viewing angles within a stadium.
A continuing need therefore exists for a visible line marking system that is simple to operate, accurate enough to allow its use by officials at sporting events, and of sufficient intensity to be viewed by players, large audiences, and television viewers alike.
A need also exists for a system capable of projecting a variety of other images, onto surfaces having non-uniform light scattering properties, which can be seen from different perspectives and from considerable distances even in daylight conditions.
The aforementioned needs are addressed, and an advance is made in the art, by an apparatus for providing at least one temporary visible reference line on a surface, as for example, an athletic field, within the field of view of at least one video camera. An illustrative system constructed in accordance with the present invention comprises a first laser source disposed at a first elevated, stationary location relative to the surface, the first laser source being operative to emit a first laser beam having a wavelength of between 400 nm and 750 nm and to sweep the first laser beam along a selectable path upon the surface so as to form a temporary line thereon. The system further comprises a second laser source disposed at a second elevated, stationary location relative to the surface and different from the first stationary location, the second laser source being operative to emit a second laser beam having a wavelength of between 400 nm and 750 nm and to sweep the second laser beam across the selected path so as to form, with the first laser beam, a composite temporary visible line as, for example, a line of demarcation during a football game. A controller comprising a synchronization module is operatively associated with the first and second laser sources and is adapted to synchronize the sweep frequency ratexe2x80x94at which the first laser beam and the second laser beam are swept across the surface of the fieldxe2x80x94to a scan rate of at least one camera that is arranged to receive an image of the composite temporary line.
In accordance with an illustrative NTSC-compliant embodiment of the present invention, the sweep frequency rate of the beams and the camera scan rate are both 30 Hz. Alternatively, the camera scan rate may be a sub harmonic of the sweep frequency rate. Assuming NTSC compliant operation, the sweep frequency rate may be an n multiple of the 30 Hz camera scan rate, where n is a whole integer greater than one. In a professional or collegiate football setting, such synchronization allows television viewers to enjoy a flicker-free display of the same visible line as that seen by the players, game officials and fans in attendance. The synchronizing module can be adapted either to synchronize the scan rate of the at least one camera to the sweep frequency rate of the first and second laser sources, or to synchronize the sweep frequency rate of the first and second laser sources to a scan rate of the first and second laser sources.
It is expected that the power delivery requirements for each laser source can vary considerably for each installation, depending upon such variables as the range of expected ambient lighting conditions, the distance each beam must traverse before contacting the surface, and the actual width dimension of the line to be displayed. For a line width of approximately 3 to 6 inches (8 to 15 cm), excellent results have been achieved from distances in excess of several hundred feet using two 40W, frequency doubled, Q-switched Nd:YAG lasers each adapted to generate laser pulses at a wavelength of 532 nm. Emission at this wavelength is especially preferred since it is very close to the peak (555 nm) of the human eye""s sensitivity. By comparison, in an argon ion laser operating in continuous wave (cw) mode, roughly half of the output is at 514 nm (58% as bright as the same beam at 555 nm), another 30% is at around 480 nm (18% as bright) and the remaining 20% is at around 440 nm (barely visible to he human eye). Thus, such an argon laser would have to deliver up to three or four times as much power to match the visibility of an Nd:YAG laser.
To move its respectively generated laser beam along a selectable path on a target surface so as, for example, to produce a straight line connecting the two lateral sides of a football field, each corresponding laser source includes a scanning assembly capable of deflecting the beam as required to generate the composite line. By way of illustrative example, the scanning assembly for each laser source may include an X-Y galvanic scanner. The controller is responsive to user input commands to move the composite temporary line from a first selectable path on the surface to a second selectable path on the surface. Where the installation is to provide a line of demarcation in a football game, this can include moving the temporary visible line forward from a first down line of scrimmagexe2x80x94where an official has just xe2x80x9cspottedxe2x80x9d the ballxe2x80x94by a distance of ten yards to thereby display the new first down line. This can also include moving the temporary visible line from an old line of scrimmage, forward or backward, to a new line of scrimmage as a result of a penalty assessed against one of the teams.
Under certain ambient lighting and other installation conditions, it is contemplated that a surface may be divided into multiple regions or zones. This allows the distance over which each beam must travel to be kept within a range that is consistent with the intensity, divergence and line width demands for proper viewing and accuracy. By way of illustrative example in which the surface is a football field that is subject to bright daylight illumination conditions, the first and second laser sources may be positioned beyond and above opposite lateral sides of the 25 yard line on one-half of the field, while third and fourth laser sources may be positioned beyond and opposite lateral sides of the 25 yard line on the other half of the field.
It should be emphasized that there is no requirement that any pair of lasers be located along a line transverse and perpendicular to the lateral sidelines of the field. Thus, for example, the first laser source might be beyond and above the twenty-yard line of a first lateral side of the field and the second laser source might be beyond and above the thirty-yard line of the second lateral side. Still another laser source might be at the ten-yard line of either the first or the second line, such that all or any two of them may be used to generate the composite line. Preferably, however, the various laser sources are arranged so as to cause the incident light from different beams to be scattered in a way that allows spectators from as many different viewing angles as possible to see the line clearly. In that regard, there is no requirement that the respective laser sources be located at the same elevated vertical position relative to the field. Needless to say, it is considered within the level of skill of the ordinary artisan to obtain, whether empirically or by predictive modeling, a juxtaposition of laser sources that is ideally suited to the specific lighting conditions and overall dimensions associated with any particular indoor or outdoor location.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings. It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.